This five-year proposal is designed to promote a career as an academic clinician investigator in the fields of immunology and infectious diseases. The candidate will complete fellowship training in infectious diseases and be appointed to the faculty of the Emory University School of Medicine, Department of Medicine, Division of Infectious Diseases. Her career development will be mentored by Dr. David S. Stephens, Distinguished Professor of Medicine, and Professor of Microbiology and Immunology, and Epidemiology, and Director Division of Infectious Diseases. This research plan will be conducted in Dr. Stephens' laboratory which has a long and successful track record in microbial pathogenesis and molecular genetics. The Toll-like receptor 4 (TLR4), together with its accessory protein MD-2, and endotoxin are responsible for initiating much of the inflammatory response in sepsis caused by meningococci and other gram-negative bacteria. Previous work in the laboratory has identified an important role for meningococcal KDO2-lipid A in activation of the TLR4/MD-2 receptor complex. The hypothesis is that MD-2 is a specific determinant of TLR4 activation and that the KDOz-lipid A endotoxin structure is critically important for direct binding to MD-2. The importance of KDO in binding to MD-2, other endotoxin structural requirements for binding to MD-2 and the activation of TLR4 will be investigated. Specific Aim 1: Determine the meningococcal KDO2-lipid A structure required for interaction with the human adaptor protein MD-2 using genetically and biochemically defined meningococcal lipooligosaccharides. Specific Aim 2: Determine the general endotoxin structure required for extracellular activation of TLR4 via the accessory protein MD-2. Significance. Understanding the role of MD-2 in TLR4 activation by endotoxin may impact therapeutic interventions that modify the inflammatory immune response. This proposal will identify the components of bacterial endotoxin which are most efficiently recognized by MD-2 and presented to TLR4. The study of other diseases (e.g. atherosclerosis, inflammatory bowel disease, and asthma) with inflammatory components which are mediated by the activation of TLR4 and MD-2 may also benefit from this work.